1. Field of the Invention
This invention generally relates to semiconductor wafer processing systems and, more particularly, to a process for igniting a plasma to process a substrate, such as a semiconductor wafer, in a plasma processing chamber of a semiconductor wafer processing system.
2. Description of the Background Art
Substrates such as semiconductor wafers are often processed by methods involving the use of plasma in the presence of the wafer or substrate. During wafer processing, the wafer rests on a pedestal containing an electrostatic chuck that can be a monopolar, or preferably, a bipolar electrostatic chuck. Electrostatic chucks contain one or more electrodes embedded within a dielectric material such as polyimide. Such a polyimide chuck is described in commonly assigned U.S. patent application Ser. No. 08/744,039, filed Nov. 5, 1996, now U.S. Pat. No. 5,885,469, and incorporated herein by reference. When a voltage is applied to the electrodes, charges on the wafer and charges on the electrodes electrostatically retain the wafer on the chuck surface. As such, the wafer is held in a stationary position while being processed.
To accomplish wafer processing, such as plasma precleaning, a wafer is supported in a process chamber upon a pedestal. The pedestal generally contains an electrostatic chuck for retaining the wafer while the wafer is processed, e.g., exposed to a plasma to sputter clean the surface of the wafer. The chuck has one or more electrodes embedded within a chuck body. The chuck body is fabricated of a dielectric such as polyimide, aluminum nitride, alumina, and the like. In a well known manner, a voltage, applied to the electrodes, retains the wafer against the support surface of the chuck by electrostatic force.
An anode electrode is disposed above the pedestal and the pedestal generally contains a conductive element (e.g., a pedestal base) that is used as a cathode. During plasma cleaning of a wafer, a gas such as argon, helium, hydrogen, or a combination thereof is supplied to the chamber and energy is applied between the cathode and anode to produce a plasma. The active gas atoms bombard the wafer and sputter clean its surface. Typically, energy from a direct current (DC) voltage ignites and sustains the plasma. However, a radio frequency (RF) voltage may also be used to sustain the plasma and/or bias the wafer.
More specifically, in a wafer cleaning process, a reactive cleaning gas (process gas) such as hydrogen is introduced into the chamber. The plasma is formed when electrons are stripped from a portion of the process gas atoms to form positive ions. Positive ions and electrons both leak out of the plasma; however, the electrons, being lighter, move faster and therefore leave more rapidly. As a result, the plasma is at an electric potential that is positive with respect to the chamber walls, which are usually at ground potential. Electron bombardment of the surface of the wafer makes the plasma positive with respect to the wafer. This self bias of the wafer causes ions to accelerate towards the wafer and bombard its surface. If the wafer is further biased with an electric potential from a power supply that is substantially negative with respect to the plasma, additional positive ions from the plasma are accelerated towards the wafer.
While the plasma exists in the chamber, voltages on the wafer and the chuck are generally defined by the potential of the plasma. For the particular case that the wafer is allowed to "float" relative to the chamber walls and the chuck, some charge accumulates on the wafer primarily due to bombardment of the surface by energetic electrons from the plasma. However, as the wafer charges, an electric field develops which acts to repel electrons from the plasma. Thus, the potential difference between the plasma and the wafer is self limiting. The potential difference between the plasma and the floating wafer is approximated by the formula: ##EQU1##
where .DELTA.V is the potential difference between the plasma and the floating wafer, k is Boltzmann's constant, T.sub.e is the electron temperature, m.sub.i is the mass of an ion and m.sub.e is the mass of an electron. Typical values of .DELTA.V are in the range of 1 to 10 volts. This is generally small enough that field emission from the wafer does not occur during plasma processing. However, during plasma ignition, transient bursts of high voltage can cause, under some circumstances, field emission from the wafer.
A plasma in a preclean chamber is typically ignited by a transient burst of high voltage applied to the cathode electrode. The chamber, anode and chuck electrodes are typically grounded during ignition of the plasma. The transient high voltage can be larger than 1000 volts and typically lasts about 1 second, though it can last longer if there are problems achieving ignition. During this transient burst of high voltage, a similarly high transient voltage develops on the wafer. If the voltage on the wafer is large, a substantial electric field exists at the surface of the wafer. If the chuck electrodes are energized during the ignition of the plasma, the transient voltage on the wafer is even higher due to the high electric potential already existing between the chuck electrodes and the wafer. This high voltage can lead to substantial charging of the wafer through field emission. A similar effect occurs if other components within a pedestal are energized or grounded during plasma ignition including components such as a bias electrode or resistive heater. Charging of the wafer through field emission is undesirable because it leads to substantial charge imbalance between the wafer and the chuck. The charge imbalance results in a residual charge being accumulated on the chuck surface. After the electrodes are deactivated, the residual charge will retain the wafer with such force that the wafer cannot be removed from the chuck. Charging of the chuck surface can also lead to arcing between wafer and chuck that can damage the wafer and/or the chuck.
Therefore, a need exists for a method of protecting a wafer on an electrostatic chuck against charging during ignition of a plasma or other similar high voltage process.